1. Field of the Invention
The present invention relates generally to magnetic resonance imaging (MRI), also known as magnetic resonance tomography (MRT) as applied in medicine to examine patients.
2. Description of the Prior Art
MRI is based on the physical phenomenon of nuclear magnetic resonance and has been successfully used as an imaging procedure in medicine and in biophysics for over 15 years. Using this examination modality, the patient is exposed to a strong, constant magnetic field. This causes the nuclear spins of atoms in the object (which were previously randomly oriented) to align in a certain direction. Radio-frequency energy can then excite the aligned nuclear spins to a certain oscillation. It is this oscillation that generates in the MRI system the actual measurement signal, which is then received using suitable reception coils. Using spatially linearly variable magnetic fields generated by gradient coils, the examination subject can be spatially encoded in all three directions in space, which is generally called “location coding”.
The acquisition of data in an MRI system occurs in so-called k-space (frequency domain). The MRI image in the “field-of-view” is associated with the MRI data in k-space by Fourier transformation. The location coding of the object, which encompasses (fills) k-space, occurs by means of gradients in all three directions in space. In doing so, a distinction is made among layer selection (defines an acquisition layer in the object, e.g., the Z-axis), frequency coding (defines a direction within the layer, e.g., the x-axis), and phase-coding (defines the second dimension within the layer, e.g., the y-axis). In addition, during the 3D imaging the phase-coding can subdivide the selected layer in partitions, e.g., along the z-axis.
Thus, first a layer is selectively excited, e.g., in the z-direction, and—if need be—a phase coding in, e.g., the x-direction can be performed. The encoding of the location information in the layer is done by a combination of phase and frequency coding by means of the two previously mentioned orthogonal gradient fields that (in the example of the layer being excited in the z-direction) are generated by the gradient coils in the x- and y-directions.
In order to scan a whole layer of a subject to be surveyed, the imaging sequence (e.g., the gradient echo sequence, FLASH) is repeated n times for various values of the phase-coding gradient, e.g., Gy. At each sequence, the magnetic resonance signal (e.g., gradient echo signal) is scanned, digitized, and stored by a Δt-pulsed ADC (Analog Digital Converter), which is also done n times in equidistant time steps Δt in the presence of the selective gradient GF. In this manner, a number matrix is created in individual rows (a matrix in k-space, i.e., a k-matrix) with N×N data points. By Fourier transformation, this set of data allows an MR image of the scanned plane to be reconstructed with a resolution of N×N pixels (a symmetrical matrix with N×N points is only an example; asymmetrical matrices also can be generated). The values in the region of the center of the k-matrix contain primarily information about the contrast, and the values in the peripheral area of the k-matrix tend to contain information regarding the resolution of the transformed MRI image.
MRI also allows sectional images of the human body to be acquired in all directions. MRI as a sectional imaging procedure in the practice of medical diagnostics is characterized, above all, as a non-invasive examination method. In spite of this defining feature, angiographic images (i.e., images of the blood vessels in the human body, particularly those in the organs with sufficient blood supply) are limited in terms of their contrast in the generic MR imaging process; these limits, however, can be substantially overcome with the use of contrast agents. The effect of contrast agents is generally based on influencing the parameters that are decisive for the contrast, such as the longitudinal or transversal relaxation time T1 or T2. The substance most often used in clinical practice is trivalent gadolinium Gd3+, which has a T1-shortening effect. Binding in the so-called chelate complexes (DTPA, Diethylene Triamine Pentaacetic Acid) causes gadolinium to lose its toxicity so that Gd DTPA usually can be applied by means of intravenous administration. The operator selects a vein that leads directly to the heart, which then distributes the contrast agent throughout the entire arterial system—usually from the aortic arch to the toes. In the sequences that are generally used (T1-weighted spin echo sequence, gradient echo sequence, etc.), the accelerated T1 relaxation causes an enhancement of the MR signal; it is a lighter representation of the relevant tissue in the MR image. In this manner, sharp, rich-in-contrast images of, for example, head, neck, heart, or kidney vessels can be obtained.
Such procedure based on the use of a contrast agent in the magnetic resonance imaging is generally called “Contrast-Enhanced MR Angiography (CE MRA). The quality of the contrast agent based vessel images depends largely on the temporal co-ordination of the sequence steps characterizing the scanning, which is generally called timing or contrast agent timing. The basic sequence steps are: contrast agent injection, a writing time, and scanning of the center of the k-space matrix. In order to achieve the best possible contrast of the image, the center region of the k-matrix should be scanned when the contrast agent is present in the area of interest (field of view—FOV) in a maximum concentration. For this reason, according to the current state-of-the-art the contrast-enhanced angiography is performed as follows:
A contrast agent is injected intravenously into the patient's body, which then gets distributed through the heart in the arterial vessel system—especially from the aortic arch to the toes. After the contrast agent is properly distributed, the individual sections of interest of the subject are excited one after another. After a section has been scanned, the patient is shifted while lying on a movable table by the width of the section, and a new body section of the same dimensions is excited and scanned. The scanning of a 3D section of a width 10 to 15 cm lasts about 22 seconds so that the scanning of the whole body from the heart to the toes takes about 1.5 minutes.
According to the current state of the art, due to the relatively long duration of such scanning by individual sections, the gradual distribution of the contrast agent cannot be exactly monitored. This results in the sections being scanned in different phases of the contrast agent distribution, the images also reflect the contrast agent that is present in the veins. Superimposition of the venous system upon the arterial system makes an angiographic image unusable.